1. Field of the Invention
The present invention relates to CMOS image sensor design and more particularly to lowering imaging lens requirements of CMOS image sensors.
2. Background of the Related Art
There has been an increase of digital image devices using CMOS image sensors. A conventional CMOS image sensor requires a matching imaging lens unit which includes one or more lens elements to direct incoming light to its sensor surface and generate an image on the sensor. In general, the chief ray angle for a pixel at the center of the sensor surface is zero, while the chief ray angles for pixels at corners and/or edges of the sensor surface reach to a considerable amount. To mitigate the negative effects of large chief ray angles and consequently generate a uniform image over the entire sensor surface, the imaging system of the CMOS image sensor may be customized under given imaging lens requirements.
In general, the imaging lens requirements may refer to the limitations on the number of lens elements and size of the imaging lens unit (assembly). A complex imaging lens unit having multi lenses may require a complicated optical design and a high manufacturing cost. Thus, it is desirable to minimize the number of imaging lens elements without degrading the performance of the imaging system.
The overall physical dimension of the imaging system may determine the size of imaging lens unit. Thus, the lens size requirement may become more significant as the size of the imaging device decreases as in typical mobile applications. For this reason, the mobile sensor device engineers often make significant efforts to minimize the lens size without compromising the performance of the imaging system.
However, the conventional approaches to change the number of lens elements and size of the imaging lens unit in an effort to lower the lens requirements may encounter additional difficulties as these requirements may be accompanied by a considerable amount of alteration and customization of the entire CMOS image sensor layout. Thus, there is a need for an improved methodology that lowers the lens requirements without modifying the overall imaging system significantly and compromising the image quality and performance of the sensor.